Hydrocarbons can be produced through a wellbore traversing a subterranean formation. The wellbore may be relatively complex. For example, the wellbore can include multilateral wellbores and/or sidetrack wellbores. Multilateral wellbores include one or more lateral wellbores extending from a parent (or main) wellbore. A sidetrack wellbore is a wellbore that is diverted from a first general direction to a second general direction. A sidetrack wellbore can include a main wellbore in a first direction and a secondary wellbore diverted from the main wellbore and in a second general direction. A multilateral wellbore can include a window to allow lateral wellbores to be formed. A sidetrack wellbore can include a window to allow the wellbore to be diverted to the second general direction.
A window may be an opening in a sidewall portion of a casing string. The window can be pre-milled by being created before the casing string is positioned in the wellbore. Casing strings with pre-milled windows can be used to reduce or eliminate debris. Aluminum outer sleeves can be positioned outside of the pre-milled windows to prevent debris from entering the inner diameter of the casing string through the pre-milled windows during positioning of the casing string in the wellbore, or otherwise. After a casing string is positioned in the wellbore, an aluminum outer sleeve can be milled to allow the branch wellbore to be drilled.
Before creating branch wellbores, the parent wellbore can be completed. Completion methods can include fracturing the formation in proximity to a production zone of the parent wellbore by pumping fracturing fluids into the well at high pressure to stimulate hydrocarbon production from the formation. Other completion tasks can include the introduction of high pressure. Casing strings can also experience high pressure in the wellbore independent of the high pressure introduced into the wellbore. Aluminum or similar types of outer sleeves may need to be relatively thick to retain a general configuration and to withstand burst and collapse pressures.
Thick aluminum sleeves increase the outer diameter of casing strings. In some applications, the outer diameter may be increased by one or more inches. Such an increase in the outer diameter can be unacceptable in some situations.
In some applications, the outer sleeves are glass fiber and a steel inner sleeve is positioned inside the casing string to provide support. The steel inner sleeve, however, needs to be retrieved to complete the wellbore, or otherwise to form the branch wellbore. Retrieving the steel inner sleeve can require a separate run and can be costly.
Therefore, an assembly is desirable that can provide sufficient support for a pre-milled casing string window and avoid requiring a substantial increase in the outer diameter of the casing string. Assemblies are also desirable that withstand burst and collapse pressures and avoid substantially increasing outer diameters of casing strings. Assemblies are also desirable that do not require a separate run to retrieve an inner sleeve.